The objectives of this proposal are: 1) detect early focal changes in mitotic activity (MA) of rat mammary epithelium associated developmentally with preneoplastic hyperplastic alveolar nodules (HANs); 2) isolate tissues with such abberant MA from the mammary glands of carcinogen-treated and old rats, and test by transplantation techniques the hypothesis that they are developmental precursors of HANs; 3) study the biological properties of the presumptive HAN precursor (PHP) to ascertain how they differ from normal and HAN tissue; and 4) establish an experimental model, based on the above, which can be used to study ways of aborting neoplasia in its early stages from progression to more advanced stages. MA of normal Lewis/Mai rat mammary tissue will be studied to establish normal limits. Methods will include administration of colchicine to arrest mitoses, tissue samples at defined intervals in the estrus cycle, preparation of whole mounts of inguinal mammary gland, and microscopic analysis of the latter for the extent and topologic distributon of MA. Data will be in the form of frequency distributions of lobulo-alveolar (LA) structures with different degrees of MA for each stage of the sexual cycle, and for rats of different ages. The mammary glands of carcinogen-treated rats will be similarly analyzed to search for foci of altered MA. The frequency distribution of LA structures with different degrees of MA will be compared to those of age and cycle matched controls. The frequency of HANs in the carcinogen-stimulated gland will be determined and correlated with any changes in the frequency distribution of LA MA. From the above analysis we should be able to recognize the PHP. PHP will then be isolated along with control tissue, and established as transplantation lines in cleared fat pads. The transplant lines will be analyzed for the frequency with which they give rise to HANs. The biologic properties of PHP tissue will be studied and compared to derivative HAN tissue. Methods will include organ culture, mitotic analysis, 3H-TDR autoradiography, growth response to hormones, and in vivo antigenicity studies.